Immunoglobulin E (IgE) is an important regulator of allergic reactions, in which it activates mast cells (MCs) by binding to its high-affinity receptor FcεR1 (Kinet J P. Annu Rev Immunol. 1999; 17:931-972). IgE is the least abundant antibody isotype in humans, and its role in human immunology (other than its effects on allergy and parasitic infection) has long been unclear. In addition to MCs, lymphocytes, dendritic cells, eosinophils, platelets, monocytes, and macrophages also bear FcεR1 on their surfaces, albeit in different assemblages. For example, FcεR1 on MCs is a heterotetramer (αβγ2), whereas FcεR1 on macrophages or eosinophils is a heterotrimer (αγ2). In dendritic cells, the expression of FcεR1 affects IFN-γ-mediated pro-inflammatory (tumor necrosis factor-α [INF-α]) and anti-inflammatory (IL-10) cytokine production, as well as the efficiency of antigen uptake and presentation.